Optical fiber Raman lasers are known. See, for instance, U. S. Pat. No. 5,323,404. All herein cited references are incorporated herein by reference. Briefly, in a Raman laser, a cascaded Raman resonator (CRR) receives pump radiation of wavelength .lambda..sub.p (typically from a laser diode array and a cladding pumped fiber laser), and shifts radiation power in one or more steps to a desired output wavelength .lambda..sub.s &gt;.lambda..sub.p. The step size (frequently referred to as the Raman shift) typically is about 13.2 THz, corresponding to the maximum in the Raman gain spectrum of silica. The step size is determined by reflectors (typically refractive index gratings) in the optical fiber.
FIG. 1 schematically depicts an exemplary prior art CRR 10. The pump source can be conventional (e.g., comprising a diode laser array and a cladding pumped fiber laser) and is not shown. In FIG. 1, pump light of wavelength .lambda..sub.p propagates from fiber 11 to fiber 12 through fibers 13, 14 and 15, all single mode silica-based fibers. Fiber 15 typically is of order 1 km in length, and advantageously has low effective area to maximize Raman gain. Numerals 161, 162-16n are conventional high reflectivity (e.g., &gt;90%) refractive index gratings. Downstream gratings 171, 172 . . . 17n are matched in center wavelength to upstream gratings 161 . . . 16n, thereby providing resonance cavities at the wavelengths that correspond to the respective center wavelengths. One of gratings 171-17n (e.g., 17n) is a low reflectivity grating, with all others having high reflectivity. By way of example, the pump wavelength .lambda..sub.p is 1117 nm, the output wavelength .lambda..sub.s. is 1480 nm, and the intermediate Stokes-Raman orders are 1175, 1240, 1310 and 1395 nm, corresponding to the center wavelengths of the respective grating pairs. FIG. 2 shows the measured Raman gain spectrum for .lambda..sub.p =1.mu.m. See G. P. Agrawal, "Nonlinear Fiber Optics", 2nd edition, Academic Press, 1995.
The prior art CRR of FIG. 1 comprises an optional high reflectivity pump reflector grating 18 (with center wavelength .lambda..sub.p), as well as a low reflectivity (e.g., about 5%) output grating of center wavelength .lambda..sub.s. By way of example, the grating pair 161/171 corresponds to the first intermediate Stokes-Bragg order, and the pair 16n/17n corresponds to the output wavelength. In a CRR as described, the light of an intermediate Raman-Stokes order circulates in its resonator until the light is substantially entirely converted into the next Raman-Stokes order. FIG. 3 of the '404 patent shows an exemplary spectrum of a prior art CRR.
It will be understood that the arrangement of the various refractive index gratings is not critical, since in general there is relatively little interaction between light and a grating of the type relevant herein unless the wavelength of the light is essentially equal to the center wavelength of the grating. See co-assigned U.S. patent application Ser. No. 08/871,023, filed Jun. 6, 1997 by Reed et al., which discloses that appropriate ordering of the reflectors can result in conversion efficiency improvement of order 1%.
Conventionally, the pump radiation of wavelength .lambda..sub.p is derived from a high power pump that typically comprises a commercially available diode laser array and a cladding pumped fiber laser that serves to convert the multimode radiation from the diode laser array into single mode radiation of the appropriate wavelength.
CRRs as described above are finding a variety of uses, e.g., providing 1480 nm pump radiation for EDFAs, or providing pump radiation for Raman amplification of signal wavelengths of about 1.55 .mu.m from 1100 nm pump light.
In principle it is possible to provide a dedicated Raman laser for each desired output wavelength. See, for instance, K Rottwitt et al., OFC98, San Jose, Calif. However, provision of a multiplicity of Raman lasers, one for each desired wavelength, typically would be prohibitively expensive. Thus, it would be desirable to have available a Raman laser that produces output radiation of two or more predetermined wavelengths from a single pump wavelength .lambda..sub.p. This application discloses such a multi-wavelength Raman laser.